No Windows Presentation Foundation support is one thing some would miss. Seriously though, there's a lot of clients out there that would probably not miss the Microsoft branding if they got over the .net label missing. Perhaps not larger enterprise clients that rely on specific MS tools and support, but a lot of smaller businesses could leave it just fine that use it for things like asp.net and switch to apache + mono.

To be fair though, the parent I was replying to did not define in what capacity .net would be replaced and how and taking replies as a black and white answer to a gray area question is a bit factitious (this part was not meant as a directly reply to the parent of this reply).